USB ports are found on many portable devices such as laptop computers, tablets, mobile phones, MP3 players, etc., and are also provided on desktop computers, automobile dashboard consoles, etc., to provide interconnection for serial communications between devices. The USB standards provide for charging capability by which mobile phones or other portable devices can be operated using power provided through a USB cable to the device. This power feature of the USB system also advantageously allows battery-powered devices to be charged using power provided from a connected USB-compatible device. Dedicated charging equipment is available, for example, having multiple USB ports for charging various portable devices, even where no serial communication is needed between the charging equipment and the device. As portable device functionality expands and as customer expectations as to operating time between charges increases, device battery capacity has been enhanced, but this also increases the time to fully charge the battery or batteries of an electronic device. Faster charging can be achieved by increasing the charger efficiency and/or output power level. Switching power supply architectures such as synchronous switching buck converters can achieve high efficiency, for example, up to 95%, and thus increasing the charger power level is the primary means of reducing charging times for portable electronic devices. However, proper device operation requires correct matching between the power levels of the charging source and the charged device.
Proprietary adapters have been developed, typically providing a fixed voltage level as a charging source for fast charging requirements of designated portable device. However, the fixed voltage provided by proprietary adapters may not be suitable for charging certain devices. Accordingly, this approach typically requires use of proprietary mechanical and/or electrical interfaces to ensure only target portable equipment can be charged, thereby preventing universal usage. A continuing problem for fast battery charging is therefore how to provide high input power to a device while maintaining universal charging capability. USB adapters are the most common universal power source for portable battery-powered electronic devices such as smartphones and tablet computers, where the USB Battery Charging standards allow different portable devices to share the same power source. The original USB implementations provided for charging at a nominal voltage level of 5V with a charging current limited to 1 A, and subsequent revisions to the standards (e.g., USB 3.0, 3.1, etc.) provide for fast charging at higher levels, thus accommodating up to 100 W charging. However, mismatching between USB charging sources or charging adapters and charged devices can hamper proper device operation and/or prevent the ability to minimize charging time. The USB-PD standard provides a communication interface with the device modulating a high frequency signal on the VBUS power line to allow communications with the USB adapter, but this approach requires filter circuitry to demodulate control signals and limits the utility in mobile phone and other cost sensitive applications. Accordingly, there is a need for improved universal USB charger apparatus and electronic devices by which the charging power level can be maximized without degradation of the charger or charged electronic device for fast charging.